Hydrotalcite is a naturally occurring mineral having the formula 6MgO.multidot.Al.sub.2 O.sub.3 .multidot.CO.sub.2 .multidot.12H.sub.2 O or Mg.sub.6 Al.sub.2 (OH).sub.16 CO.sub.3 .multidot.4H.sub.2 O. Known deposits of natural hydrotalcite are very limited and total only about 2,000 or 3,000 tons in the whole world. Natural hydrotalcite has been found in Snarum, Norway and in the Ural Mountains. Typical occurrences are in the form of serpentines, in talc schists, and as an alteration product of spinel where, in some cases, hydrotalcite has formed as pseudomorphs after spinel.
The upper stability temperature of hydrotalcite is lower than the lower limit for spinel. Spinel and hydrotalcite theoretically never would appear together in stable condition. If equilibrium has been established, the spinel would be completely changed to hydrotalcite. However, naturally occurring hydrotalcite is intermeshed with spinel and other materials.
Natural hydrotalcite is not present as pure product and always contains other minerals such as penninite and muscovite and potentially undesirable minerals such as heavy metals. Conventional practice recognizes that it is practically impossible to remove such impurities from a natural hydrotalcite.
Previous attempts to synthesize hydrotalcite have included adding dry ice or ammonium carbonate (a) to a mixture of magnesium oxide and alpha-alumina or (b) to a thermal decomposition product from a mixture of magnesium nitrate and aluminum nitrate and thereafter maintaining the system at temperatures below 325.degree. C. at elevated pressures of 2,000-20,000 psi. Such a process is not practical for industrial scale production of synthetic hydrotalcite by reason of the high pressures. Furthermore, the high pressure process forms substances other than hydrotalcite, such as brucite, boehmite, diaspore, and hydromagnesite.
There is increasing interest in anion substituted hydrotalcite. U.S. Pat. No. 4,883,533, issued to Kosin et al discloses a modified synthetic hydrotalcite. The patent teaches forming a synthetic hydrotalcite from a slurry of Mg(OH).sub.2, sodium aluminate and NaHCO.sub.3. The synthetic hydrotalcite is then reslurried in water with phosphoric acid. The acid was added until all of the carbon dioxide was liberated.
Canadian Patent 1,198,675 issued to Buehler, discloses a method of forming hydrotalcite-like complexes which include Mg.sub.6 Al.sub.2 (OH).sub.14 (HPO.sub.4).multidot.4H.sub.2 O and Mg.sub.6 Al.sub.2 (OH).sub.14 (SO.sub.4).multidot.4H.sub.2 O. The material is formed by first forming a hydrotalcite structure from a suspension including aluminum hydroxide and magnesium hydroxide and then intercalating with either K.sub.2 HPO.sub.4 or NaHSO.sub.4.
It is an object of the present invention to produce synthetic anion substituted hydrotalcite in high purity by direct synthesis from alumina and magnesia as opposed to the indirect synthesis where hydrotalcite is first formed and then the anions substituted in the hydrotalcite structure (indirect synthesis).
It is another object of this invention to produce synthetic anion substituted hydrotalcite in high yield at atmospheric pressure.
Yet another object of this invention is a low cost method for producing synthetic anion substituted hydrotalcite.
Another object of this invention is a method for producing synthetic anion substituted hydrotalcite which does not require calcining of hydrotalcite to remove its carbonate.
Still another object of this invention is a method for producing synthetic anion substituted hydrotalcite which does not require reslurrying hydrotalcite to intercalate the anion.
Additional objects and advantages of the present invention will be more fully understood and appreciated with reference to the following description.